Book - Physiology of the Fetus 1

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Windle WF. Physiology of the Fetus. (1940) Saunders, Philadelphia.

1940 Physiology of the Fetus: 1 Introduction | 2 Heart | 3 Circulation | 4 Blood | 5 Respiration | 6 Respiratory Movements | 7 Digestive | 8 Renal - Skin | 9 Muscles | 10 Neural Genesis | 11 Neural Activity | 12 Motor Reactions and Reflexes | 13 Senses | 14 Endocrine | 15 Nutrition and Metabolism | Figures

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Chapter I Introduction

If one may judge from the number of articles published, there has always been lively scientiftc interest in the newborn infant. Not only have the functions of its various org-ans been studied but its behavior has been investigated extensively in order to try to understand normal and abnormal mental processes in the growing individual and the adult. But for the most part, birth has stood as a barrier to our lcnowledge of the genesis of physiologic functions beyond which 1itt1e penetration has been made, save by specula.tion, until receht years. Not alone to the obstetrician, pediatrician and the· chi1d psychologist is lcnowledge of prenatal function and behavior a subject of importance. Physiology of the fetus bears much the same relationship to adult physiology as structura1 embryology bears to the anatomy of the adult and should be a serious concern of all.

In casting about among the scattered accounts of observations pertaining to physiology during development one finds a wide choice of experimental Material. Many forms of animals have been studied. We are particularly concerned with conditions encountered in mammals, especially the true mammals. Most noteworthy investigations have employed fetuses of the ordinary laboratory and domestic animals and man, but less common species have been used occasionally. Observations in the opossum and in lower vertebrates, se, birds, reptiles, amphibia and lishes, have contributed very signiHcant information on function during de— velopment. It is always necessary to proceed cautiously in« at— tempting to interpret human fetal behavior in terms of results obtained in lower forms. Activities engaged in by immature individuals of any animal forecast the adult physiology of that particular species and are less distinctly related to others. They are often influenced by very speciaI conditions peculiar to the species and not so evident in higher or lower forms. Even among the true mammals there is no uniformity of fetal behavior. In the first place it varies in respect to the manner in which respiration, nutrition and elimination are provided. signikicant difkerences in placentation must be given consideration.

Relation of Fetal to Maternal Organism

The fertilized ovum requires several days for transit down the uterine tube to the uterus in most mammals and, during the while, it undergoes development from a single ce1l to many cells.

Fig. 1. Showing the sequence of ovulation, fertilization and implantation in man (Dickinson, R. L.: "Human Sex Anatomy," Williams and Wilkins, Publishers).

An efficient p1acental mechanism is unnecessary to meet the requirements of this ear1y growth. About ten days elapse between fertilization and implantation in man and in this period the ovum forms into a f1uid Hlled blastocyst not a great deal larger than the single unfertilized cell (Fig. 1). All mammals begin to deve1op a placenta at about this stage to provide nourishment and oxygen for further growth. But the efficiency of exchange between fetus and mother varies great1y and the functional activities which one encounters in studying the fetus are inHuenced correspondingly.

Intimacy of relationship between fetal and maternal tissues is unequally established in the uteri of different mammals. A graded series can be arranged on the basis of the number of tissues which separate the fetal blood from that of the mother.1 In marsupials, such as the opossum, only a simple contact is formedz the outer chorionic surface of the blastocyst rests against the unbroken epithelium of the uterus. This is the least eflicient relationship, one in which materials must pass through two layers of capillary endothelium, two layers of epithelium, two layers of connective tissue and the potential lumen of the uterus in their course from one organism to the other. Formation of chorionic villi characterizes the union of fetus with the uterus in all the true mammals, but this does not in itself offer great improvement in efficiency unless some of the tissue barriers are brolcen down. In the horse and pig a diffuse arrangement of villi prevails in a sims ple epithelio—chorial placenta in which there is no erosion of maternal tissues. The villi simply make contact with the uterine epithelium and are bathed in secretions and transudates which form the "uterine milk" (Fig. a, d).

The ruminating mammals such as cattle, sheep and deer, have developed a little better arrangement in the placenta. The chorionic villi dip into the uterine glands and an incomplete brealcs down of the uterine epithelium occurs between villi (Fig. a, B) . Consequently there are regions in the cotyledonary syndesmochorial placenta of these animals wherein exchange is eflkected through one less layer of tissue than in the pig. But contact between fetus and host is quite simple and there is no tearing of the maternal tissues when the two separate at birth.

The first true deciduate placenta appears in carnivores, such as the dog and cat. Erosion of the uterine mucosa is effected by some means not at all understood although assumed by many to be by ferments liberated in the chorionic epithelium. The fetal blood stream coursing in the chorionic villi is separated from the maternal stream by the, tissues of the villi and by« the maternal capillary endothelium (Fig. 2, c) . Tearing of uterine mucosa is encountered at birth when this endothelioschorial placenta comes away.

Fig. 2. Diagrammatic Sections through the placentas of (A) the pig, (B) the cow, (C) the cat, and (D) man. (Arey: "Developmental Anatomy.")

In the hemo-chorial placentas of man and the other primates as well as insectivores and lower rodents the process of erosion of uterine mucosa by the chorionic villi results in the opening of maternal blood channels.

• Bartelmez2,3 has reported that the intervillous space of the human and monlcey placentas eontains little blood. He suggests that an active flow of tissue fluid from the neighboring maternal mpillaries tills this space from which it is expelled by rhythmical contractions of the uterus.

Consequently fetal blood is separated from that of the mother by only the tissues of the villi themselves (Fig. 2, D). An even more eflicient hemo-endothelial placenta has been developed by the higher rodents, such as the rat, guinea pig and rabbit,4 and-in it the fetal capillaries are separated from maternal blood by no tissues other than their own endothelium. Placental classilications compiled from studies of Grosser and Mossman are summarized in Table 1.

Table 1 Tissues and Substances Separating Fetal from Maternal Blood in Five Types of Placentas

Type of placenta	Epithelio-chorial	Syndesmo-chorial	Endothelio-chorial	Hemo-chorial	Hemo-endothelial
Maternal tissue:
Endothelium	+	+	+	-	-
Connective	+	+	-	-	-
Epithelium	+	-	-	-	-
Uterine milk or pablum	+	+	-	-	-
Fetal tissue:
Chorion	+	+	+	+	-
Mesenchyme	+	+	+	-	-
Endothelium	+	+	+	+	+
Examples	Horse, swine	Cattle, sheep	cat, dog	Man, monkey	rabbit, guinea pig, rat
Data from

lt must be remembered also that there is a certain degree of recapitulation of placental history during developmenr. The rabbit whose fully formed placenta is of the hemo-endothelial type has essentially a hemo—chorial or perhaps even a syndesmo-chorial placenta earlier, as will be seen in Table z. A yollosac placenta is often present as a transient supplementary -structure, but it may persist and function concurrently throughout gestation as has been demonstrated in the rat.0i 7 .

Time and again in the study of prenatal physiology it will appear that species variations in the intimacy between maternal and fetal blood streams may explain difkerences in experimental results. Perhaps some of these are real but others seem to be artikactua1. Opening the Uterus and exposing a fetus with an epithelio—chorial placenta, whose contact with the host is slight, can be expected to disturb respiratory exchange and other functions more readily than the same procedure in one with a hemochorial or hemoendothelial placenta The fundamental species differences in placentation have not often been given adequate consideration in attempting comparative interpretation of data.

Table 2 Tissues Separating Fetal from Maternal Blood in the Rabbit Placenta at Various Ages

Age in days	8	10	17	28
Maternal tissue:
Endothelium	+	-	-	-
Connective	+	-	-	-
Epithelium	-	-	-	-
Fetal tissue:
Chorion	+	+	+	-
Mesenchyme	+	+	-	-
Endothelium	+	+	+	+

Functional variations in early prenatal like may be related to other dikkerences in development of the ketal membranes. For example, the allantois, aside from playing an important part in providing vascularization of the ectoplacenta, is concerned to a variable extent in storing and absorbing ketal excretions. In many mammals including man this organ is of little consequence because of its rudimentary nature. But in others, such as the pig, sheep and cat, it is highly developed. lt has been suggesteds that those with large allantoic vesicles have large kunctional mesonephros and inadequate provision kor early placental elimination, a concept which has recently been questioned.

lt is known that the activities of the uterus itselk bring about changes in prenatal function Disturbance of normal uterine tone alters the behavior of the fetuses. Thus we Find that immobilization of the uterus during the late stages ok gestation by unbalancing its hormonal control makes possible the observation of somatic activities characteristically seen earlier during conditions ok anoxemia.10 There is some evidence that the rhythmical contraction and relaxation of the uterine musculature which is found toward the end of pregnancy serves to vary the oxygenation of the fetal blood.U A few attempts have been made to correlate uterine motility with changes in other physiologic functions of the fetus but much remains to be determined.

The normal physiology of the uterus and placenta must be considered in choosing an animal for investigation in fetal physiology. But these are by no means the only factors. It is important to take account of the rate of embryonic growth, which may be so rapid that a gradcd fctal sekjes at the various functions! stages is ditiicult to assemble. Some species are more mature at birth than others: contrast the guinea pig with the newborn kitten for example. Lengths of gestation of the principal forms to be considered will be found in Table 2.

Table 1 Gestation Periods of Mammals in Which Fetal Studies Have Been Made

Species	Duration	Species	Duration

species Dotation species Dotation Opossum . . . . . . . . . . . . . . . 13 days Fig . . . . . . . . . . . . . . . . 17 vseelcs

Mouse . . . . . . . . . . . . . . . . . 20 «« sheep . . . . . . . . . . . . . . 21 ««

Rat. . . . . . . . . . . . . . . . . . . . . 21 « Goal; . . . . . . . . . . . . . . . 21 ««

Rabbit . . . . . . . . . . . . . . . . . 82 «« Monlcey (Maeae). . 24 ««

Dog . . . . . . . . . . . . . . . . . . . 68 «« Man . . . . . . . . . . . . . 38 ««

cui; . . . . . . . . . . . . . . . . . . .. 65-67 «  40 «

Guinea pig . . . . . . . . . . . . . 67269 «« klorse . . . . . . . . · . . . . . . 48 ««

Fetal structural dilkerences account for many of the kunetionai variations which are encountered in studying different species of animals. For example, one can not expect to observe the development of the characteristically anthropoid hand and foot reflexes in the embryos of hoofed animals. However, it is often more important to use fetuses of large size than small ones whose morphology and placental physiology are more similar to man. But here again it may be« impossible to draw deductions concerning such functions as those of the endocrine glands in the human fetus from experiments in the cow or sheep whose placentas are less permeable to molecules of large size than is that of man. consequently, selection of the experimental material must be made critically to fit the problem under consideration.

Experimental Methods

Fully as important as se1ection of suitable specimens for the investigation is the employment of an experimental method which will interfere as 1ittle as possible with normal functions of both maternal and ketal organisms Many of the earlier studies in fetal behavior appear to have been conducted without beneiit of anesthesia,I·«’ a practice whose use today is limited to the most minor surgical procedures. It is usually essential that the pregnant anis mal be rendered unconscious and this can be accomplished by one of a number of methods.

Some experiments can be conducted under a general anesthetic such as ether, urethane or one of the barbituric acid compounds. However, it is very important to consider the action of these drugs upon the uterus and the fetus and to decide whether or not and to what extent the functions to be investigated will be influencecl by them. In many instances this has not been done and results have consequently been wrongly interpreted.

A local anesthetic may be satisfactory to insure quiescence of the maternal animal when given either in the form of cutaneous inliltration with cocaine solutions or as an intradural injection. Some species lend themselves to such treatment better than others but there is always the danger, especially with cutaneous injection, of the anima1s becoming restless and interfering with observations in the fetuses at a critical moment.

Some investigators have preferred to Section the spinal cord at a high level and under a general anesthetic preliminary to malcing observations in the fetuses. It is essential to allow sufhcient time to elapse after the operation not only for recovery from the anes— thesia but to overcome the effects of shock. When this has been done the animals, although conscious, can be operated upon with— out further treatment because sensations are no longer perceived from the region below the spinal transection and the specimens are immobilized by paralysis.

A method which in many respects is more satisfactory for ketal studies is that involving anemic decerebrationsls It has been used most successfully in cats but has likewise been adapted to ratsI4 and can be used in other species with certain moditications The animals are Erst anesthetized with ether. The carotid at— teries are ligated high in the neclc making certain that the blood flow to the brain from these sources is stopped. A tracheal cannula is inserted to facilitate the next stage in the operation. After incising and retracting the soft palate and mucous membrane on the roof of the pharynx, a small hole is drilled through the cranium between the two tympanic bullae and the dura mater is torn to allow the cerebrospinal fluid to escape. A ligature is then passed around the basilar artery at about the middle of the pons and this vessel is tied. This method of decerebration renders the animals unconscious without signiiicant loss of blood and with— out seriously disturbing other physiologic functions. There is no evidence of surgical shoclc, for continuous records of the animals’ blood pressures talcen during the operation show no depressionJs A variable degree of decerebrate rigidity appears when the ether is discontinued after the operation. Respiration is usually slowed but increases in rate during the course of an hour or more which should be allowed for complete recovery from the anesthetic. As a rule the rate is normal or faster than normal by the time of ex— perimentation. Consciousness is lost, the animals usually are unresponsive to manipulation of the viscera and there is no further need for anesthesia.

Even under the best of experimental conditions it is exceeds ingly diilicult if not actually impossible to expose and maintain fetuses in a state comparable with that in utero. An attempt is usually made to deliver them without separating the placenta from the -uterine wall and some investigators have assumed that by doing so respiration and nutrition of the specimens are preserved at normal levels. This is seldom the case because efliciency of the placental exchangebecomes seriously impaired by changing the spatial and pressure relationships with incision of the uterus and subsequent removal of part of its contents. Furthermore, it seems probable that the less intimate the contact between maternal and fetal parts of the placenta, the greater are the chances that opening the uterus will upset physiologic conditions in the fetuses. These facts must be faced and results interpreted accordingly.

It is essential to realize iirst of all that the behavior exhibited by fetuses at experimental hysterotomy, even when the placentas are allowed to remain attached, is not. that occurring within the undisturbed uterus but is afkected by the pärtial anoxemia attendant upon the experimental procedureLs However, in many instances the purpose of the experiment is to determine what functions the fetus is capable of performing and not what it may actually do. consequently it« is not essential to preserve the placental mechanism in perfect physiologic condition for long.

The problems involved in most investigations in physiology of the fetus are complicated because one is dealing with two organisms maintaining mutual although precarious relationships to one another. It is true that the fetus· cannot be studied under physiologic conditions when the health of the mother is jeopard· ized, but on the other hand the best of conditions in the mother do not insure that behavior of the extracted fetuses will always be normal. Only by understanding the fundamentals of physiology of respiration, circulation, metabolism, etc., in the adult can one hope to arrive at signiftcant information concerning similar functions in prenatal life. Gradually problems occasioned by the inaccessibility of the fetus which have baflled investigators in the past are being solved through the invention of new and ingenious technical procedures.

References Cited

Brunschwig, A. E. i927. Anat. Rec., 34: 237.

Evereth J. W. i935. J. Exp. Zool» 7o: 243.

. Bremer, J. L. i9i6. Am. Anat., i9: i79.

. Gersh, I. i937. contr. Emb., as: II«

. Windle, W. F., M. Monnier sc A. G. steele. i938. Physiol. Zoöl., it: 425. . Windle, W. F. sc A. G: steele. ig38. Proc. soc. Exp. Biol sc Med., Zy 246.

is. Preyen W. i885. specielle Physiologie des Embryo- Stichen. Leipzig. i s. Pollocly L. J. sc L. E. Das-is. i924. Arch. Neur. Psychiat., is: 288.

i4. Windle, W. F. sc W. L. Mineaix i933. Anat. Rec., 57: i.

is. Windle, W. F. sc R. F. Becken ig4o. Arch. Neur. Psychiat., 43: 9o.

i6. Dicltinson, R. L. i933. I-1uman sex Anatomy, Williams sc Wilkins, Bal timore (ref. for Fig. i) . U. Arey, L. B. i94o. Developmental Anatomy, saunders, Philadelphia (ref. for Fig. s) .

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